It all depends.
- There is no penalty if you're getting a refund, provided you file within the allotted 3-year timeframe.
- After 3 years, the "penalty" is forfeiture of your tax refund, as mentioned above.
- There is no penalty if you filed an extension and paid any additional taxes owed by April 15, as long as you file your return by the October 15 deadline.
- A late filing penalty applies if you owe taxes and didn't file your return or extension by April 15.
- This penalty also applies if you owe taxes, filed an extension, but didn't file your return by October 15.
- The late filing penalty is 5% of the additional taxes owed amount for every month (or fraction thereof) your return is late, up to a maximum of 25%.
- Tip: The late filing penalty is 10 times higher than the late payment penalty. If you can't pay your tax bill and didn't file an extension, at least file your return as soon as possible! You can always amend it later.
- A late payment penalty applies if you didn't pay additional taxes owed by April 15, whether you filed an extension or not.
- The late payment penalty is 0.5% (1/2 of 1 percent) of the additional tax owed amount for every month (or fraction thereof) the owed tax remains unpaid, up to a maximum of 25%.
Example: Let's say you didn't file your return or extension by April 15, and you still owe the IRS an additional $1,000.
Best-case scenario: You file your return on April 29, 2 weeks late, and submit your payment for $1,000. You would owe an additional $50 for filing late ($1,000 x .05) plus another $5 for late payment ($1,000 x .005) for a total penalty of $55.
(Had you filed your extension by the deadline, your total penalty would only be $5. It pays to file an extension!)
Worst-case scenario: You file your 2012 return in April of 2018, 5 years late, and submit your payment for $1,000. You would owe an additional $250 for filing late ($1,000 x the maximum .25) plus another $250 for late payment ($1,000 x the maximum .25), for a total penalty of $500.